feat: add Stirling PDF toolkit

Added Stirling PDF as the 4th tool in the kit:

📄 Stirling PDF Features:
- 50+ PDF operations (merge, split, convert)
- OCR processing for scanned documents
- Digital signatures and watermarks
- Privacy-first: locally hosted, no cloud dependency
- Open source with 69.6k+ GitHub stars

🎨 Visual:
- Purple-blue gradient matching brand
- PDF document icon with lines
- Badges: Privacy, Open Source, Free

📊 Updates:
- Stats section: 3 → 4 Tools
- Footer: Added Stirling link
- README: Added tool description
- Grid layout: Now displays 4 tools responsively

🔗 URL: https://stirling.kit.pivoine.art

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -134,6 +134,7 @@ Tailwind CSS 4 uses a new CSS-first configuration approach:
 - **Vert** - Privacy-focused file converter (images, audio, documents)
 - **Paint** - Browser-based image editor
 - **Pastel** - Modern color manipulation toolkit with palette generation and accessibility testing
+- **Stirling** - Powerful PDF toolkit with 50+ operations (merge, split, convert, OCR, sign)
 
 ## CI/CD Pipeline